1. Field of the Invention
The present invention is directed to policies for improving throughput in remote mirroring systems, such as peer to peer remote copy.
2. Description of the Related Art
Disaster recovery systems typically address two types of failures, a sudden catastrophic failure at a single point in time or data loss over a period of time. In the second type of gradual disaster, updates to volumes on data storage may be lost. To assist in recovery of data updates, a copy of data may be provided at a remote location. Such dual or shadow copies are typically made as the application system is writing new data to a primary storage device. International Business Machines Corporation (IBM), the assignee of the subject patent application, provides several remote mirroring systems, including, for example: a synchronous Peer-to-Peer Remote Copy (PPRC®) service and a PPRC® Extended Distance service in an Enterprise Storage Server (ESS) system.
The synchronous PPRC® service provides a technique for recovering data updates that occur between a last, safe backup and a system failure. Such data shadowing systems can also provide an additional remote copy for non-recovery purposes, such as local access at a remote site.
With the synchronous PPRC® service, a primary storage subsystem maintains a copy of predefined datasets on a secondary storage subsystem. The copy may be used for disaster recovery. Changes to data are copied to the secondary storage subsystem as an application updates the data. Thus, the copy may be used whether there are gradual and/or intermittent failures. The copy is maintained by intercepting write instructions to the synchronous PPRC® dataset and generating appropriate write instructions from the primary storage system to the secondary storage system. The write instructions may update data, write new data, or write the same data again.
The synchronous PPRC® service copies data to the secondary storage subsystem to keep the data synchronous with a primary storage subsystem. That is, an application system writes data to a volume and then transfers the updated data over, for example, Enterprise System Connection (ESCON®) fiber channels to the secondary storage subsystem. The secondary storage subsystem writes the data to a corresponding volume. Only when the data is safely written to volumes at both the primary and secondary storage subsystems does the application system receive assurance that the volume update is complete.
With synchronous PPRC®, the copy at the secondary storage subsystem is maintained by intercepting write instructions to the dataset at the primary storage subsystem and generating appropriate write instructions from the primary storage system to the secondary storage system.
PPRC® Extended Distance service does not write to secondary storage subsystem before acknowledging the write to the primary. Instead, for the PPRC® Extended Distance service, when a track is written, information is stored that indicates that the track is to be transferred to the secondary storage subsystem at a later time. An asynchronous process collects updates at the primary storage subsystem and sends the updates to the secondary storage subsystem.
In some cases, a first track is removed from cache at the primary storage subsystem to make room for a second track before the first track is written to the secondary storage subsystem. In such a case, the first track is later read back into cache at the primary storage subsystem and then is transferred to the secondary storage subsystem. Removing the first track from cache and later reading the first track back into cache for transfer is inefficient. Thus, there is a need in the art for cache policies for improved throughput in remote mirroring systems, such as peer to peer remote copy.